The semiconductor industry's drive to continually improve performance and density has forced the use of advanced materials and interconnect structures. High interconnect performance requires the reduction of resistance and capacitance. Copper metallization was introduced in 1998 to reduce the resistance of interconnect wiring. Capacitance reduction or the introduction of low dielectric constant insulators, herein referred to as low-k dielectrics, are needed for future performance enhancements.
For over 25 years, silicon dioxide has been the dielectric insulator of choice for the semiconductor industry. Silicon dioxide possesses excellent dielectric breakdown strength, a high modulus, good thermal conductivity, a low coefficient of thermal expansion, and excellent adhesion to metallic liners, plasma enhanced chemical vapor deposited (PECVD) barrier cap layers, and other like materials. However, with reduced ground rule dimensions and the need for improved performance, silicon dioxide is slowly being phased out and replaced with materials possessing lower permitivity to achieve reduced capacitance. For example, at the 180 nm technology node, fluorosilicate glass is replacing silicon dioxide in many applications.
At the 130 nm technology generation, “true” low-k dielectrics are being implemented into semiconductor products. There are several candidate materials but the industry has focused primarily on two material classes: spin-on polymers and carbon-doped PECVD silicon dioxide dielectrics.
Polymer dielectrics may be used as insulating layers between various circuits as well as layers within circuits in microelectronic devices, such as integrated circuits, multichip modules, and laminated circuit boards. The microelectronics fabrication industry is moving toward smaller geometries in its devices to enable lower power and faster speeds. As the conductor lines become finer and more closely packed, the requirements of the dielectrics between such conductors become more stringent.
While polymer dielectrics often provide lower dielectric constants than inorganic dielectrics, such as silicon dioxide, polymer dielectrics often present challenges to process integration during fabrication. For example, to replace silicon dioxide as a dielectric in integrated circuits, the polymer dielectric must be able to withstand processing temperatures during metallization and annealing steps of the process. Preferably, the dielectric material should have a glass transition temperature greater than the processing temperature. The dielectric must also retain the desirable properties under device use conditions. For example, the dielectric should not absorb water which may cause an increase in the dielectric constant and dielectric loss and which may potentially lead to corrosion of metal conductors.
Porous thermoplastic polymers, particularly thermally stable polymers, such as polyimides, have also been investigated for use as low-k alternatives to silicon dioxide. Although such porous thermoplastic materials can be made to have acceptable dielectric constants and are relatively tough, being able to withstand the mechanical processing steps necessary to fabricate microelectronic devices, the pores tend to collapse during subsequent high temperature processing thereby precluding porous thermoplastic polymers for applications of interest.
Another class of low-k polymers that are attracting considerable interest in the microelectronics industry is thermosetting resins, particularly polyarylene resins. Such thermosetting resins are disclosed, for example, in WO 98/11149. Specifically, WO 98/11149 discloses dielectric polymers, which are the reaction products of a cyclopentadienone functional compound and an acetylene functional compound.
Although thermosetting resins are available, it has been determined that such resin formulations may suffer from pore collapse when attempting to form a porous structure from the resin by introducing a porogen into the b-staged formulation, thereby rendering such porous thermosetting resins also unsuitable for use in many microelectronic applications.
The prior art literature in this field may be divided into a number of different classes. The first is the formation of bloomers that are functionalized with acetylenic substituents that are capable of inducing chain extension or crosslinking during processing. The crosslinking may be thermal or promoted by a catalyst. These prior art references are characterized by oligomeric materials containing acetylenic substituents bound to the oligomers. The prior art falling into this group include, for example, U.S. Pat. Nos. 4,587,315, 5,493,002, 5,426,234, 5,446,204, 6,265,753, 5,268,444 and 5,312,994.
There are a number of patents such as, for example, U.S. Pat. Nos. 6,288,188, 6,252,001 and 6,121,495, that describe the preparation of b-staged solutions of polyarylenes prepared from different intermediates and subsequent thermosetting after processing. While the matrix materials are similar, the reactive functionality (type and number) is determined by monomer stoichiometry before b-staging.
U.S. Pat. No. 6,093,636 describes the preparation of porous organic thermosets including polyarylene compositions by the thermal labile porogen approach. These porous organic thermosets may suffer from pore collapse.
U.S. Pat. Nos. 5,426,234, 5,446,204, 5,268,444 and 5,312,994 describe the use of various acetylenic reactive diluents which react with functionalized oligomers. In these prior art references the goal was to lower viscosity for polymer melt processing and to increase the crosslinking density after curing. This technique was applied to the formation of dense thermosetting polymers with no mention of porosity.
U.S. Pat. No. 6,359,091 describes a polyarylene composition in which the thermosetting resin does not undergo a significant drop of modulus at temperatures above 300° C. during curing. That feature reportedly enabled one to form porous films by avoiding pore collapse. Specifically, the '091 patent discloses that by modifying the formulations so that the resins do not undergo a significant drop in modulus during cure or alternatively shifting the temperature at which the minimum modulus occurs to a lower temperature enables one to avoid pore collapse. Thus, in the '091 patent the modulus-temperature profile is modified such that the modulus drop of the b-staged resin prior to network formation is minimized. In the '091 patent, a crosslinker is added to a polyarylene oligomer solution.
U.S. Pat. No. 6,468,589 describe a composition for film formation which comprises a poly(arylene ether) polymer having repeating structural units represented by formula (1) mentioned at Col. 2, lines 25-34. The poly(arylene ether) polymer is made by heating a bisphenol compound and a dihalogenated compound in a solvent in the presence of a catalyst such as an alkali metal compound. Crosslinking agents such as actylenes may be present in this prior art composition.
In view of the state of the art mentioned hereinabove, there is still a need for providing thermosetting polymer compositions which are capable of providing a low-k dielectric material having nano-sized pores which are substantially stable and do not collapse during further high temperature processing
In the present invention, a low molecular weight compound that plasticizes the resin and acts as a reactive additive is employed in forming thermosetting polymer compositions. During the thermal ramp to the cure temperature, one role of the additive of the present invention is to create a situation where the glass transition temperature of the materials advances to a temperature above the actual cure temperature of the system, i.e., to modify the classical Time Temperature Transformation profile for the thermoset. This aspect of the present invention is not described in any of the above mentioned references. A characteristic of many organic thermosets is that the glass transition temperature of the thermoset closely tracks the cure temperature. In the present application, by using the inventive additive, the applicants have unexpectedly determined that a condition is created where the glass transition temperature of the thermoset increases to a temperature above the cure temperature, provided a threshold temperature is exceeded. This feature is not mentioned in any of the prior art mentioned above.